A known shock absorbing structure for a vehicle disclosed in for example JP2002-155980A (US2002060463A1) includes a bumper reinforcement and a pair of side members extending in a longitudinal direction of the vehicle, each of which is located at end of a bumper reinforcement. Further, a crash absorbing box serving as a shock absorbing device is provided between the bumper reinforcement and each of the side members. This crash absorbing box is consecutively plastic deformed (buckling deformed) so as to be in a concertina shape in order to absorb the impact energy.
For example, one known crash absorbing box is comprised of a pair of pressed iron material in the shape of square bracket with corners cut off in cross section, and these materials are put together and welded so as to be hollowed and have a rectangular cross section.
Another known crash absorbing box is comprised of a flat iron plate and a pressed iron material in the shape of square bracket with corners cut off in cross section, and these materials are put together and welded so as to be hollowed and have a hexagonal cross section.
Because the crash absorbing box hollowed and has a rectangular cross section or a hexagonal cross section has small number of the ridge lines extending in an axial direction, in other words, the number of peak points in its cross section of the crash absorbing box is small, the thickness of the crash absorbing box needs to be increased in order to increase a level of the load (axial compression load) required for deforming the crash absorbing box.
Further, because the length of each side of the crash absorbing box in cross section is long, a wave length of the load (axial compression load) required when the crash absorbing box is consecutively plastic deformed (buckling deformed) so as to be in a concertina shape in order to absorb the impact energy, becomes large, and then amplitude of the load (axial compression load) becomes large, as a result, the energy absorbing effect is reduced. The energy absorbing effect can be calculated by the formula; (energy absorbing amount)/(maximum load×maximum stroke). In this formula, the maximum stroke means a stroke at a point where, even when the load is applied, the crash absorbing box is no longer deformed. In addition, the energy absorbing amount means a total of the load absorbed by the crash absorbing box until the stroke becomes a maximum value, in other words, the energy absorbing amount means an area which is closed by each of lines and the x-axis in FIG. 6.
Structures of the crash absorbing box being hollowed and having a rectangular cross section and a hexagonal cross section and an energy absorbing manner of the crash absorbing box upon an axial compression obtained on a basis of an experimental test will be explained.
FIG. 10A illustrates a front view indicating the crash absorbing box 91 being hollowed and having a rectangular cross section.
As shown in FIG. 10A, the crash absorbing box 91 includes a pair of pressed iron materials 91a and 91b. Specifically, these materials formed has an approximate C-shaped cross section with each corner makes a right angle, and these materials are put together and welded so as to be in a rectangular shape in its cross section. A length of the long sides of the crash absorbing box 91 is set to 87 mm, and a length of the short sides of the crash absorbing box is set to 59.6 mm. Further, a thickness of the crash absorbing box 91 is set to 2.3 mm, and a length of the crash absorbing box 91 in an axial direction (in a direction perpendicular to FIG. 10A) is set to 114.2 mm.
Furthermore, on the top end portion of the crash absorbing box 91 in an axial direction, a stress concentrated portion (vulnerable portion) is formed to be a starting point for the plastic deformation caused by an axial compression load. The stress concentrated portions are formed in order to reduce the axial compression load by which the crash absorbing box 91 starts plastic deformation (initial buckling deformation).
A dashed line in FIG. 6 indicates a relationship between a deformation characteristic (stroke) and an axial compression load based on an experimental test in which a predetermined energy amount J (joule) is applied to the crash absorbing box 91 and the bumper reinforcement 16 so as to be compressed in an axial direction. A range in which the bumper reinforcement has been deformed is also shown in FIG. 6.
As shown in FIG. 6, the crash absorbing box 91 being hollowed and having a rectangular cross section performs large wavelength and large amplitude of the load (axial compression load) required for deforming the crash absorbing box when the crash absorbing box 91 has consecutively repeated plastic deformation (buckling deformation) so as to be in a concertina shape in order to absorb the impact energy.
Further, it is experimentally confirmed that the energy absorbing effect, which is obtained until the stroke reaches the maximum stroke, is low such as 65%.
FIG. 10B illustrates a front view indicating the crash absorbing box 92 being hollowed and having a hexagonal cross section.
As shown in FIG. 10B, the crash absorbing box 92 includes a flat iron plate 92a and a pressed iron material 92b formed so as to be in a square bracket with corners cut off in cross section. Specifically, these materials are put together and welded so as to be in a rectangular shape in its cross section, and each of the welding portions makes a right angle. Further, a long side of the pressed iron material 92b is located so as to be perpendicular to each of the short sides of the pressed iron material 92b. 
Assuming that each corner of the pressed iron materials 92b exist, a length of the long side is set to 115.7 mm, and a length of each of the short sides is set to 62.5 mm. Further, a thickness of the iron plate 92a is set to 2 mm, and a thickness of the pressed iron material 92b is set to 1.6 mm, and a length of the crash absorbing box 92 in an axial direction (in a direction perpendicular to FIG. 10B) is set to 232 mm.
Furthermore, on the top end portion of the crash absorbing box 92 in an axial direction, a stress concentrated portion (vulnerable portion) is formed to be a starting point of the plastic deformation caused by an axial compression load.
A chain double-dashed line in FIG. 6 indicates a relationship between a deformation characteristic (stroke) and an axial compression load based on an experimental test in which a predetermined energy amount J is applied to the crash absorbing box 92 and the bumper reinforcement 16 so as to be compressed in an axial direction.
As shown in FIG. 6, the crash absorbing box 92 being hollowed and having a hexagonal cross section performs small wavelength and small amplitude of the load (axial compression load) required for deforming the crash absorbing box. However, it is experimentally confirmed that the energy absorbing effect, which is obtained until the stroke reaches the maximum stroke, is low such as 80%.
A need thus exist to provide a shock absorbing device for a vehicle and a shock absorbing structure for a vehicle which can be reduced in size and can improve an energy absorbing effect.